DETERMINATION OF ACIDITY AND ALKALINITY 



101 



No definite sudden rise is observed in this case. The "equivalence 

 point" Hes at about pH 11.5. If a very weak acid of the general type of 

 boric acid is to be titrated it can only be done in the following way: 

 A suitable indicator, such as ?/i-nitro-benzene-azosalicylic acid 

 (Alizarine Yellow GG), is added to a pure solution of primary sodium 

 borate, and an equal amount of the same indicator is added to the 

 boric acid solution; the latter is then titrated until the same color as 

 that in the sodium borate solution is reached. Since the rise on the 

 pH curve through the point of equivalence is very small, the color 

 change of the indicator is very gradual, and the titration could barely 

 be carried out without the aid of a standard for comparison. 



Q 1 Z 3 US 6 7 a 9 10 n 12 13 n 15 16 17 18 19 20 Z1 22 

 ccm n NaOH 



Fig. 14. Titration curve of a mixture consisting of 10 cc. A'' HCl + 10 cc. 

 A'' acetic acid. 



The limits of error of such titrations vary with the conditions. 

 A theory of titration error has been advanced by Bjerrum.' This 

 theory may be roughly summarized by stating that the steeper the 

 rise of the pH curve through the "point of equivalence" the smaller 

 the error. 



In connection with the above it is interesting to observe the behav- 

 ior on titration of a mixture consisting of a strong and of a weak acid. 

 Let us titrate a mixture containing 10 cc. of A^" HCl and 10 cc. of N 

 acetic acid in a volume of 100 cc. with N NaOH. The pH curve of 

 this titration will follow the course shown in figure 14. 



The calculation of the pH values is carried out as follows : 



1. As long as free HCl is present it suppresses almost completely 



' See footnote 7. 



